1. Field of the Invention
The present invention relates to a hydrogen storage electrode and a process for producing the same. The hydrogen storage electrode is employed as an anode of an alkaline secondary battery in which hydrogen is utilized as an anode active material. The present invention particularly relates, for instance, to a process for producing a hydrogen storage electrode enabling to produce a large-sized electrode easily and intending to improve the electric discharging characteristic of an electrode.
2. Description of the Prior Art
A metal oxide-hydrogen battery has been known so far as one of alkaline secondary batteries in which the metal oxide is utilized as a cathode active material and the hydrogen is utilized as an anode active material. As one of the metal oxide-hydrogen batteries, there has been a battery including and utilizing a hydrogen storage electrode, which reversively absorbs and releases hydrogens, as an anode active material.
The following have been known as processes for producing such a hydrogen storage electrode:
1. A sintering method in which a hydrogen storage alloy is sintered onto an electric current collecting substrate.
1.1 In one of the sintering methods, a TiNi-Ti.sub.2 Ni alloy is employed. (Journal of the Less-Common Metals, 104 (1984) 365-373)
1.2 In another one of the sintering methods, an AB.sub.2 type alloy such as ZrMn.sub.0.6 Cr.sub.0.2 Ni.sub.1.2 is employed. (U.S. Pat. No. 4,728,586, U.S. Pat. No. 4,716,088 and European Patent Application No. 0293660A2)
The sintering method can be applied to the above-mentioned tenacious alloys, but it is not an appropriate method for a brittle alloy such as a lanthanum-nickel alloy (LaNi.sub.5), a mischmetal nickel alloy (MmNi.sub.5) and the like.
2. A mixing method in which a hydrogen storage alloy is mixed with a binder, i.e., polytetrafluoroethylene (hereinafter referred to as "PTFE"), polyvinyl alcohol (hereinafter referred to as "PVA"), sodium carboxylmethyl cellulose (hereinafter referred to as "CMC") and the like, for instance.
2.1 A hydrogen storage alloy powder is mixed with a PVA aqueous solution, thereby making a paste. The paste is then filled in a 3-D electrode substrate such as foamed nickel and the like. (Japanese Unexamined Patent Publication (KOKAI) No. 233967/1986, European Patent No. 0271043A1, and H. Ogawa, M. Ikoma, H. Kawano and I. Matsumoto, in Proc. 16th Int. Power Sources Symp., Bournemouth, September 1988, p. 393)
This method suffers from an expensive cost of the 3-D electrode substrate. In addition, the electrode is usable in a seald battery, but the hydrogen storage alloy powder comes off sharply when the thus made electrode is used in a vented battery electrolyte.
2.2 A hydrogen storage alloy powder is mixed with a fluororesin powder, and made into pellets. The pellets are hot-pressed onto an electric current collecting substance at a high temperature of approximately 300.degree. C. (Japanese Unexamined Patent Publication (KOKAI) No. 64069/1986 and Japanese Unexamined Patent Publication (KOKAI) No. 101957/1986)
The following are the problems of this method: It is hard to make a large-sized electrode, and it requires the high temperature.
2.3 A hydrogen storage alloy powder is compounded with PTFE, and made into a sheet or a paste. The sheet or paste compound is press-bonded onto an electric current collecting substance. (Japanese Unexamined Patent Publication No. 16470/86, European Patent No. 0284063A1 and European Patent No. 0266162A2)
The following are the problems of this method: The PTFE is expensive, and the binding strength is not enough. Accordingly, the hydrogen storage alloy powder comes off sharply due to the repetition of charge and discharge.
2.4 A hydrogen storage alloy powder is mixed with a copper powder by an amount as much as 5 to 10 times by weight of the hydrogen storage alloy powder. The mixture is then molded under pressure. (H. Buchner, Energiespeicherung in Metallhydriden, Springer Verlag, Wien and New York 1982 and J. J. G. Willems, Philips J. Res., 39 (Suppl. 1) (1984) 1)
The following are the problems of this method: The energy density decreases sharply, and the bidning strength is not enough. Accordingly, the hydrogen storage alloy powder comes off sharply.
Although the hydrogen storage electrodes favorably absorb and release hydrogens, they should exhibit a low electric resistance. Accordingly, the hydrogen storage electrodes are molded after mixing a hydrogen storage alloy powder and a binder. However, as the charge and discharge cycle is repeated and the number of the charge and discharge cycles is accumulated, the hydrogen storage alloy powder is turned into fine particles. As a result, the capacities of the hydrogen storage electrodes deteriorate and the hydrogen storage electrodes themselves break up. Therefore, it is important to take extra care in the selection of the binder.
As good examples of the conventionally known binders, and as set forth in the paragraphs 2.2 and 2.3 above, the Japanese Unexamined Patent Publication (KOKAI) No. 101957/1986 discloses the fluororesin powder as the binder, and further the Japanese Unexamined Patent Publication No. 16470/1986 discloses the PTFE powder as the binder. These publications disclose a method in which the surface of a hydrogen storage alloy powder is coated with copper to make microcapsule, the microcapsule is compounded with the fluororesin powder (the binder), and the mixture is pressed and bound onto an electric current collecting substance to make a hydrogen storage electrode.
However, there have been the following problems even in the fluororesin bound type hydrogen storage electrodes disclosed in the above-mentioned prior art publications: The hydrogen storage alloy powder is turned into fine particles, thereby deteriorating the capacity thereof as the accumulation of the charge and discharge cycles, and the capacity is deteriorated sharply during a rapid electric discharge or a high rate electric discharge. The binder amount may be increased in order to overcome the problmes or improve the binding strength, i.e., the configuration stability. Whereby the configuration stability can be upgraded, namely the hydrogen storage alloy powder can be prevented from turning into fine particles and coming off, and whereby the disadvantage of the capacity deterioration, which results from the accumulation of the charge and discharge cycles or the cycle life deterioration, can be improved. However, if the binder amount is increased, the hydrogen storage alloy powder amount is decreased and the permeation of the battery electrolyte and the hydrogen ions is hindered. Accordingly, the electric resistance of the hydrogen storage electrode is increased and the capacity thereof is deteriorated during a high rate electric discharge.
Hence, it has been desired to make a hydrogen storage electrode having an excellent cycle life and being superior in a high rate electric discharging characteristic without increasing the binder amount or while increasing the binder amount as less as possible.